yarrow

Question

The base of the stem of my yarrow plants are dead looking and have turned black ,and the leaves are dead. The top portion of the stem is green and the plant is still blooming. Should I cut the plant back or wait until Fall?

Answer 1

This happens quite often in my garden as well. The lack of air circulation at the base of the plant is the cause of this darkening and die back.
If you don't mind a bit of a messy garden, leave until the flower is done. If you want to clean up the garden, go ahead and cut the plants back.
In my own garden Yarrow can be a bit invasive, so I'm pretty ruthless with the plants. If they flop over or start looking a bit rough, I yank them right out.

https://www.gardeningknowhow.com/edible/herbs/yarrow/growing-yarrow.htm